Electromagnetic transducer for use in deep water



May 9, 1967 F. MASSA, JR 3,319,220

ELECTROMAGNETIC TRANSDUCER FOR USE IN DEEP WATER Filed Oct. 29, 1965 2 Sheets-Sheet l I N VEN TOR.

FRANK MASSA JR.

13 Y ATTORNEY F. MAssA, JR 33%,220

ELECTROMAGNETIC TRANSDUCER FOR USE IN DEEP WATER May 9, 1967 2 Sheets-Sheet 2 FIG. 3

Filed Oct. 29, 1965 INVENTOR. FRANK MASSA JR.

ATTORNEY United States Patent Mass.

Filed Oct. 29, 1965, Ser. No. 506,904 22 Claims. (Cl. 34012) This application is a continuation-in-part of my copending application Ser. No. 457,701 filed May 21, 1965.

This invention relates generally to improvements in electroacoustic transducers and more particularly to new and improved transducers for generating very high power in very deep water at frequencies in the lower or midaudible range.

I have found that the use of low-frequency underwater electromagnetic transducer of the inertial type, as described in US. patent application, Ser. No. 34,731, filed June 8, 1960, now Patent No. 3,225,326, and US. patent application, Ser. No. 164,010, filed Jan. 3, 1962, now Patent No. 3,260,990, in combination with the novel underwater horn as described in said copending application, Ser. No. 457,701, produces very great increases in output acoustic power. For example, a transducer structure approximately 22" diameter, operating in combination with a horn, approximately 5 ft. long and 4 ft. diameter mouth opening, increases the radiated acoustic power over twenty times for the same amplitude of vibration of the transducer in the 400 cycle region. This very great increase in acoustic output results from the increased acoustic loading presented to the transducer by the horn. Because of the effectiveness of the horn loading, I find that the acoustic power generating capability of the inertial type electromganetic transducer is no longer limited by its amplitude of vibration, but the limitation is generally one of electromagnetic drive force which is needed to overcome the increased loading imposed by the horn.

Some inertial type electromagnetic transducers with which this application is primarily interested are shown in FIG. 1 of US. patent application, Ser. No. 34,731, which is a balanced armature type construction, and in FIG. 1 of US patent application, Ser. No. 164,010, which is a permanent magnet polarized structure. In both these types of transducers, as well as in my improved transducer, which is disclosed in this invention, the operation of the transducer results from the mechanical vibrations induced in the sealed outer housing by the forces generated between a first magnetic-conducting means which is rigidly coupled to the outer housing, and a second magnetic-conducting means which is flexibly suspended from the first magnetic means. The second magnetic means is part of a massive inertial member which is also flexibly suspended from the sealed outer housing. Appropriate air gaps and electromagnetic driving arrangements are provided, as shown in the two referenced applications for causing electromagnetic alternating force to be generated between the inertial member and the outer housing during the application of alternating current to the drive coils.

One of the limitations of both the balanced armature design, as shown in US. application, Ser. No. 34,731, and the permanent magnet design shown in US. application, Ser. No. 164,010, is that a portion of the magnetic circuit which establishes the operating air gap is rigidly attached to an inner portion of the radiating area of the vibrating housing. When such a transducer is subjected to very high hydrostatic pressures the strains that are produced in the housing are transferred to the magnetic assembly in a manner such as to cause a change in the air gap dimension with resulting variations in performance in extremely deep water. In the case of the balanced magnetic design illustrated in FIG. 1 of U8. patent application, Ser. No.

34,731, the air gap will be reduced at high pressures both by the bending strain in the fiat end plates as well as an additional component of strain produced by the compressive stress transferred by the end plates to the cylindrical housing. In the permanent magnet design illustrated in FIG. 1 of US. patent application, Ser. No. 164,010, the change in air gap at high pressures results from the bending of the plate member which is attached to an inner portion of the spherical vibrating surface of the housing, and carries a portion of the magnetic lamination assembly.

According to an important feature of the invention, a transducer design is provided which includes the advantages of the permanent magnetic design, together with the advantages of the balanced armature drive, and, in addition, achieves complete isolation of the magnetic elements which drive the outer housing from the strains produced in the housing when operating at high hydrostatic pressures. This new design permits the vibratory magnetic forces generated in the transducer to be efficiently coupled to the outer housing, whereas strains produced in the outer housing when the structure is exposed to high hydrostatic pressures are not transferred to the magnetic assembly, thus producing the desirable result that the air gap does not vary with depth and the operation of my improved transducer is limited only by the strength of the wall of the housing structure which may be designed to withstand any submerged depth as great as the deepest known points in the worlds oceans.

Additional important features of the invention relate to the use of an inertial type transducer in combination with a horn structure to obtain a number of advantages, and to particular construction of the horn structure to obtain efficient operation at high power levels in deep water.

One object of this invention is to provide an electromagnetic transducer of improved construction which is capable of generating increased acoustic power.

Another object of this invention is to produce an improved transducer design in which the strains imposed on the housing structure, when subjected to great hydrostatic pressures, are not transferred to the air gap in a manner that would cause a change therein with a resulting change in performance characteristics.

Another object of this invention is to produce a highpower electromagnetic underwater transducer having an increased electromagnetic force generating capability whereby increased power handling capacity is achieved.

A still further object of this invention is to provide a new and improved electroacoustic sonar transducer with permanent magnetic polarization such that highefficiency operation results in the lower audio-frequency ranges.

Still another object of this invention is to provide an assembly of a transducer and a horn to obtain efficient operation at high power levels in deep water.

These and other objects of the invention are set forth with particularity in the appended claims. However, for a better understanding of the invention itself, together with further features and advantages thereof, reference is made to the accompanying description and drawings in which are shown an illustrated embodiment of the invention.

FIG. 1 is a vertical section taken through a transducer assembly including an inertial transducer and a horn in accordance with this invention;

FIG. 2 is a sectional view taken along the line II--II of FIG. 1, the view representing a section taken through the inertial transducer;

FIG. 3 is a sectional view taken along the line III-III of FIG. 2; and

FIG. 4 is a sectional view taken along line IV-1V of FIG. 2.

Reference numeral generally designates a transducer assembly constructed in accordance with the principles of this invention and particularly designed for operation in deep water to be exposed to high hydrostatic pressures.

The transducer assembly 10 comprises a horn structure 11 having an inertial type transducer 12 mounted in a throat portion thereof. Important features of the invention reside in the construction of the transducer 12 which is usable by itself as well as in combination with the horn structure, while additional features of this invention reside in the combination of the inertial type transducer 12 with the horn structure 11 and in the particular form of the horn structure.

Considering first the construction of the inertial type transducer 12, reference numeral 14 designates a rigid disc or plate having two parallel plane surfaces 15 and 16. A shell-like housing is provided including two cup-shaped housing members 17 and 18, preferably semi-spherical in form, which are attached to seal the peripheral surfaces of plate member 14. The housing members are fastened with bolts 20. O-rings 21 are provided in the conventional manner, as illustrated in FIG. 2 to effect a highpressure seal between the end surfaces at the brims of housing members 17 and 18 and the flat surfaces 15 and 16 of plate 14. The assembly of the housing members 17 and 18 to plate 14 provides a hollow spherically-shaped shell which is rigidly attached to the rigid plate which forms an equatorial plane through the sphere.

Over the central area of each surface 15 and 16 of plate 14 is rigidly bonded a stack of magnetically-conducting larninations 22 which contain an even number of slots, as illustrated in FIG. 2. The laminations are bonded to each other and the assembly in turn is bonded to the plate 14 with epoxy or other suitable cement. Within each pair of slots of the lamination assemblies 22 is assembled a coil 23. The coils 23 are illustrated in the plan view in FIG. 4 and shown schematically in the crosssectional view in FIG. 2. The coils 23 are formed of insulated electrical conductors and are rigidly bonded into the slots of assemblies 22 using a hard-setting potting compound such as epoxy, and are electrically series connected by conductors 24 shown at the ends of the coils in FIG. 4 and illustrated schematically in the cross-sectional view of FIG. 2. The two groups of coils assembled on each side of plate 14 are electrically-connected by thein-sulated conductor 25 which passes through a hole in plate 14. Conductors 26 and 27 connect the terminal ends of the entire group of the series-connected coils to electrical terminals 28 and 29. The terminals 28 and 29 are mounted to an insulating block 30 which is in turn attached to the plate member 14 as illustrated in FIG. 2. To the terminals 28 and 29 are soldered a pair of conductors 31 which run along the inside wall of housing member 18 and are electrically connected to the insulated terminals 32. An underwater cable 33 molded to a flange member 34 provides sealed terminal connections through the conductors 31 which are electrically connected to the terminals-32 in the counter-bored portion of the housing member 3', as illustrated in FIG. 2. An O-ring seal is provided to seal the surface of flange member 34 to the mating flat surface which is machined on the outer portion of housing member 18 as shown.

To complete the electromagnetic assembly for the transducer, two massive base members 37 and 38, each having a fiat plane surface, are provided with an assembly of magnetic lamination stacks 39 and 40 being provided. Between the lamination stacks 39 and 40 are bonded a number of permanent magnets 41 as illustrated in FIGS. 2 and 3. As viewed in FIG. 2 the lefthand faces of the magnets 41 may be north poles and the righthand faces may be south poles. The entire assembly of the magnets 41 and'lamination' stacks 39 and 40 are rigidly bonded together and in turn bonded to the flat surfaces of members 37 and 38 with an epoxy type cement. The arrangement of the magnetic assemblies is such that each magnet is made to line up opposite a slot, as illustrated in FIG. 2. A number of springs 42 are mounted rigidly to the members 37 and 38 by means of bolts 49. At the time of assembly it is desirable to provide a thin layer of epoxy cement between the mating surfaces of springs 42 and members 37 and 38 so as to secure complete intimate air-free contact between the metal surfaces at the time of bolting the springs 42 in place. The springs 42 are located around the peripheral surfaces of the magnet and lamination assemblies 39, 40 and 41. Surrounding the periphery of the lamination assemblies 22 are provided a number of rigid spacer bars 44. The size and location of the spacer bars 44 are such that they come into exact alignment with the faces of the spring members 42. The spacer bars 44 are bonded to plate 14 with epoxy cement. Each spacer bar 44 has a pair of clearance holes 45, as illustrated in FIG. 4, through which threaded studs 46 may be inserted when desired. The studs 46 are screwed into tapped holes located in plate 14 which are concentric with the clearance holes 45 in spacer bars 44.

In order to obtain an accurate parallel air gap between the operating magnetic structures, I find it desirable to employ the following method of assembly:

(1) Assemble lamination stacks 22 and spacer bars 44 to plate 14.

(2) Machine the entire exposed ends of spacer bars 44 and the ends of the lamination stacks 22 into a smooth plane.

(3) Assemble magnetic laminations 39 and 40 together with the magnets 41 to the plane surfaces of base members 37 and 38.

(4) Assemble springs 42 to the plane surfaces of the base members 37 and 38.

(5) Machine the unattached end surfaces of springs 42 and ends of lamination stacks 39 and 44) into a smooth flat plane.

(6) Insert studs 46 through clearance holes 45 into plate member 14.

(7) Place shims 48 with clearance holes over studs 46.

(8) Drop mating sub-assembly containing the springs 42 into position, allowing the clearance holes in the springs to pass over the studs 46.

(9) Secure mated members with nuts 49.

By following the above procedure, the air gap between the co-acting portions of the magnetic assembly will be very uniform and exactly equal to the thickness of the shims 48. After the magnetic structures are assembled and the coil leads are properly connected, the outer housing members 17 and 18 are assembled to seal and contain the complete transducer, as has been previously described.

The coils are shown connected electrically in series for convenience, although it is obvious that they may be connected in parallel or series parallel without change in the principle of operation. In connecting the coils, it is important that the group on one side of plate 14 be opposite in phase from the group on the opposite side of plate 14 in order that the common current flowing through all the coils will cause magnetic forces of opposite phase to be developed at each .air gap. This will result in a balanced armature electromagnetic drive and the instantaneous magnetic force of' attraction on one side of plate 14 will add to the instantaneous electromagnetic force of repulsion on the opposite side of plate 14, and this effectively doubles the total operating electromagnetic force which operates the transducer. The stiifness of the spring members 42 is chosen of such magnitude as to produce the desired resonance frequency in combination with the mass of the vibrating portions of the transducer.

The transducer construction just described is particularly adapted to operate at high power in very deep water. The electromagnetic forces generated by the vibrating structure are transmitted through the rigid plate member lift to the peripheral edge of the spherical enclosure 17 and 18. The massive spring-suspended inner sections 37 and 38 of the transducer will serve as inertial members to cause the outer spherical shell to oscillate as a whole during the passage of alternating current through the coils. One of the requirements for successful operation of the transducer is that the thickness of plate 14 be so chosen that the resonant frequency of the plate is higher than the desired frequency of operation of the transducer. In order to secure maximum efiiciency, it is also desirable to employ a material of relatively high stiffness-to-mass ratio, and it is equally desirable to employ a material which will not corrode when immersed in ocean waters. Titanium meets both these requirements, and its use for both the shell-like housing and plate it achieves excellent performance characteristics.

By virtue of the design illustrated in this invention, the the strains produced in the outer housing when the trans ducer is subjected to high hydrostatic pressures will not be transmitted to the air gap and, therefore, the operation of the transducer is unaffected by hydrostatic pressure. All that will happen at greatly submerged depths is that the outer housing will shrink slightly in overall diameter by virtue of the high stresses to which it is subjected. This temporary change in dimension does not affect the operation of the transducer because the dimensional change is not transmitted to the air gap.

The maximum power output of the transducer is increased as the ratio of the inertial mass assembly is increased over the mass of the portion of the transducer which vibrates in unison with the outer shell; therefore it is desirable to make the total mass of the inner inertial portion of the transducer greater than the total mass associated with the outer vibrating structure.

Although the description of my improved transducer has shown an electromagnetic assembly on both sides of the center plate, it is obvious that a transducer of lower power rating can be produced if only one electromagnetic drive section is employed on one side of plate 14. The single-ended magnetic drive, although having lower power handling capacity, would still advantageously benefit from the lack of air gap change when the transducer is subjected to high hydrostatic pressures.

Although the outer shape of the transducer is shown in approximate spherical configuration, it is obvious that other shapes could be provided. For example, the outer housing might take the form of a cylindrical shell with sealed end caps at each end. The approximate spherical shape will result in minimum mass for the outer vibrating section of the transducer for a given radiating surface area, and thereby achieve maximum operating efiiciency for minimum overall weight of the transducer assembly.

To support the transducer 12 within the throat of the horn 11, the plate 14 is secured within a sleeve 50 by means of screws 51, the sleeve 50 being slidably disposed within a fixed sleeve or liner '52 fitted in an opening 53 in the horn l1. Sleeve 52 has an inwardly-turned end flange 54 abutting a shoulder 55 at the end of opening 53, and a ring 56 is secured by screws 57 to the end of the horn 11, to hold the sleeve 52 in position. To hold the sleeve 50 in position while permitting sliding oscillatory movement thereof in an axial direction, a pair of annular washers 59 and 60 are disposed between the ends of the sleeve 50 and the ring 56 and flange 54, respectively.

The horn 11 is preferably made of a non-resonant aggregate which may advantageously consist of cast concrete containing a filler of random-shaped pieces or bits of scrap metal. The bits of metal serve to increase the density of the horn structure and also serve to produce a non-homogeneous mass which will eliminate any predominant self'resonances that could occur in the bellshaped horn if the wall were fabricated of homogeneous material. By way of example, the average density may be in the general neighborhood of 150 lbs. to 300 lbs.

h per cubic foot. The existence of any horn resonances within the desired frequency of operation could cause secondary sound radiation from the horn surfaces, which would interfere with the direct radiation of sound through the horn opening.

A surface 62 which defines the passage through the horn 11 tapers from a small opening 63 adjacent the transducer 12 to a large opening 64, preferably with an exponential increase with the area increasing by constant percentage increments for equal distances along the axis of the horn 11. The large opening 64 preferably has an area greater than of the wavelength of the Waves transmitted. Surface 62 is preferably made smooth by using a hard waterproof bonding material, such as a hard epoxy varnish, to fill any pores which may be present in the concrete surface.

It should further be noted that the square root of the effective area of the vibrating surface of the transducer 12 should preferably be less than /3 the wavelength of sound being radiated.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

What is claimed and desired to :be protected by Letters Patent of the United States is:

1. In combination in an electroacoustic transducer, a rigid plate member having two opposite parallel plane surfaces, a first pair of magnetically-conducting elements, each element rigidly attached to one of said opposite parallel plane surfaces, a second pair of magnetically-conducting means each mounted in operable relationship and separated by spring members from each of said first pair of magnetically-conducting elements, electrical conducting coil means mounted in operable relationship with said first and said second magnetically-conducting means, electrical terminal means connected to said coil means, and housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member and totally enclosing said magneticallyconducting elements and associated electrical conducting coil means.

2. The invention set forth in claim 1 further characterized in that said magnetically-conducting elements attached to the plane surfaces of the rigid plate member include slots, and further characterized in that said electrical conducting coil means are rigidly assembled inside said slots.

3. The invention set forth in claim 1 further characterized in that said second pair of magnetically-conducting means include permanent magnets for establishing the polarizing flux between said first and said second pair of separated magnetically-conducting elements.

4. The invention set forth in claim ll, further characterized in that said housing means comprises two hollow hemispherical-like shells, each shell attached to the peripheral portion of said plate member, resulting in a rigid sealed outer housing structure of approximately spherical shape for the transducer.

5. The invention set forth in claim 4, further character ized in that said hemispherical shells are attached to the opposite peripheral end faces of said plate member with a slidable pressure sealing member whereby strains produced in the outer shells by immersion in deep water are not coupled in a direction perpendicular to the plane of said plate member.

6. In combination in an electroacoustic transducer, a rigid plate member having at least one face which contains a plane area, a first magnetically-conducting element rigidly attached to said plane area, a second magneticallyconducting element mounted in operable relationship with said first magnetically-conducting element, spring members separating said first and said second magneticallyconducting elements, electrical coil means mounted in operable relationship with both said magnetically-conducting elements, electrical terminal means connected to said coil means, and housing means comprising a hollow shellli ke structure mechanically attached to the peripheral portion of said rigid plate member, and totally enclosing said magnetically-conducting elements and associated electrical coil means.

7. The invention set forth in claim 6, further characterized in that said first magnetically-conducting element contains slots, and further characterized in that said electrical conducting means are rigidly assembled inside said slots.

8. The invention set forth in claim 6, further characterized in that said second magnetically-conducting element includes permanent magnets for establishing the polarizing flux between said first and said second magnetically-conducting elements.

9. The invention set forth in claim 6, further characterized in that said housing means comprises two hollow hemispherical-like shells, each shell attached to the peripheral portion of said plate member, resulting in a rigid scaled outer housing structure of approximately spherical shape for the transducer.

10. The invention set forth in claim 6, further characterized in that said hemispherical shells are attached to the opposite peripheral end faces of said plate member with a slidable pressure sealing member whereby strains produced in the outer shells by immersion in deep water are not coupled in a direction perpendicular to the plane of said plate member.

11. In combination in an elc-ctroa-coustic transducer of the inertial electromagnetic type, a rigid plate member having at least one face which contains a plane area, a first magnetically-conducting element rigidly attached to said plane area, mechanical spacers attached to said plane area and surrounding the periphery of said first magnetically-conducting elements, and characterized in that the exposed faces of said spacers lie in the same plane as the exposed surface area of said first magnetically-conductingelement, a massive inertial member having a plane area, a second magnetically-conducting element attached to the plane area of said inertial member, said second magnetically-conducting element having a plane surface which mates in operable relationship with the plane surface of said first magnetically-conducting element, spring members attached to said inertial member and surrounding the periphery of said second magnetically-conducting element, and characterized in that the exposed surfaces of said spring members lie in a common plane with the exposed surface of said second magnetically-conducting element, shims between each exposed mating surface of said mechanical spacers and said spring members whereby a uniform parallel air gap is established between said magneticallyconducting elements.

12. In combination in an electroacoustic transducer of the inertial electromagnetic type, a rigid plate member having at least one face which contains a plane area, a first magnetically-conducting element rigidly attached to said plane area, a massive inertial member having at'least one plane area, a second magnetically-conducting element rigidly attached to said plane area of said inertial member, spring members flexibly suspending said inertial member from said plate member, said magnetically-conducting elements being mated in operable vibratory relationship with each other, electrical conducting coil means mounted in operable relationship With said first and second magnetically-conducting elements, electrical terminal means connected to said coil means, a shell-like housing structure attached to the peripheral portion of said plate member, said spring-mounted inertial assembly being characterized in that its total mass is greater than the total mass of the vibratory structure which comprises said plate member and its associated first magnetically-conducting element and shell-like housing structure.

13. The method of assembling an electromagnetic transducer comprising the following steps of: (l) assembling a first magnetically-conducting element to the surface of said rigid plate member; ('2) assembling spacer bars to said rigid plate member with the bars arranged to surround the periphery of said first magnetically-conducting elcment; (3) machining the complete exposed surfaces of the spacer bars and first magnetically-conducting element such that all surfaces lie in a common plane; (4) assembling said second magnetically-conducting element to the surface of said massive inertial member; (5) assembling springs to the plane surface of said massive inertial member with the springs arranged to surround the periphery of said second magnetically conducting element; (6) machining the complete exposed surfaces of the springs and the surface of said second magnetically. conducting element such that all surfaces lie in a common plane; and (7) placing shims between machined surfaces of spacer bars and machined surfaces of springs and as semble the spring surfaces rigidly to the spacer bars whereby the intervening shims determine an accurate parallel air gap for the assembly.

14. In an electnoacoustic transducer for operation in deep water, a rigid plate member, first magnetically-conducting mcans rigidly attached to one surface of said rigid plate member, second magneticallyconducting means, spring means mounting said second magnetically conducting means in operable relationship to said first magnetically-conducting means with an air gap therebetween, elec trical conducting coil means mounted in operable rela* tionship with said first and second magnetically-conducting means for effecting vibratory movement of said second magnetically-conducting means toward and away from said one surface of said rigid plate member, and housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member and totally enclosing said magnetically-conducting means and said electrical conducting coil means, said shell-like structure being deformable under high pressure in deep water without affecting said air gap.

15. An electroacoustical transducer as set forth in claim 14, wherein said shell-like housing structure comprises a pair of cup-like sections having brim portions with said peripheral portion of said rigid plate member being mechanically held between said rim portions.

16. An electroacoustical transducer as setforth in claim 15, wherein each of said cup-like section is substantially semi-spherical.

17. In a transducer assembly for operation in deep water, an inertial type electroacoustical transducer includinga housing means defining a vibratory surface, a horn structure having a tapered passage along its length from a small opening at one end to a large opening at the other end, and means supporting said transducer with said vibratory surface in close proximity to said small opening for transmitting compressional waves from said vibratory surface directly into said small opening.

18. In a transducer assembly as defined in claim 17, said horn structure having a wall of sufficient mass and rigidify to prevent substantial vibratory movement thereof in deep water at high acoustic power levels.

19. In a' transducer assembly as defined in claim 17,

the square root of the area of said vibratory surface being less than /3 the wavelength of'the sound being radiated in the water by the vibratory surface.

20. In a transducer assembly as defined in claim 17, said horn structure being made of a rigid non-homogeneous aggregate having a density greater than the density of water.

2 1. In a transducer assembly as defined in claim 17, said inertial type transducer comprising an inertial member flexibly suspended within said housing means first magnetically-conducting means coupled to said housing, second magneti-callyco'nducting means forming part of said inertial member, and electrical conducting coil means mounted in operable relationsip with said first and second magnetically-conducting means and energizable to cause 1 oscillation of said vibratory surface.

22. In a transducer assembly as defined in claim 21, a rigid plate member having said first magnetically-conducting means rigidly attached to a surface thereof, said housing means comprising a hollow shell-like structure mechanically attached to the peripheral portion of said rigid plate member and totally enclosing said magneticallyllQ conducting means and said electrical conducting coil means.

No references cited.

RODNEY D. BENNETT, Primary Examiner. B. L. RIBANDO, Assistant Examiner. 

1. IN COMBINATION IN AN ELECTROACOUSTIC TRANSDUCER, A RIGID PLATE MEMBER HAVING TWO OPPOSITE PARALLEL PLANE SURFACES, A FIRST PAIR OF MAGNETICALLY-CONDUCTING ELEMENTS, EACH ELEMENT RIGIDLY ATTACHED TO ONE OF SAID OPPOSITE PARALLEL PLANE SURFACES, A SECOND PAIR OF MAGNETICALLY-CONDUCTING MEANS EACH MOUNTED IN OPERABLE RELATIONSHIP AND SEPARATED BY SPRING MEMBERS FROM EACH OF SAID FIRST PAIR OF MAGNETICALLY-CONDUCTING ELEMENTS, ELECTRICAL CONDUCTING COIL MEANS MOUNTED IN OPERABLE RELATIONSHIP WITH SAID FIRST AND SAID SECOND MAGNETICALLY-CONDUCTING MEANS, ELECTRICAL TERMINAL MEANS CONNECTED TO SAID COIL MEANS, AND HOUSING MEANS COMPRISING A HOLLOW SHELL-LIKE STRUCTURE MECHANICALLY ATTACHED TO THE PERIPHERAL PORTION OF SAID RIGID PLATE MEMBER AND TOTALLY ENCLOSING SAID MAGNETICALLYCONDUCTING ELEMENTS AND ASSOCIATED ELECTRICAL CONDUCTING COIL MEANS. 